PROJECT SUMMARY/ABSTRACT With improved medical care, including the introduction of successful antiretroviral therapies (ART), the age demography of HIV/AIDS has shifted so that it is projected that over half of all individuals living with HIV are now over 50 years old. A new landscape of HIV is now apparent, with a high rate of age-related comorbidities that are exacerbated by HIV infection and side effects of ART. Bone loss and resulting osteopenia/ osteoporosis and bone fracture risk are alarmingly high in individuals with HIV, especially postmenopausal women where both estrogen deficiency and HIV infection/ART contribute to inflammation-induced bone loss. Recent research implicates dysregulation of the gut microbiome and worsening gut mucosal permeability (?leaky gut?) causing influx of inflammatory mediators, such as lipopolysaccharide (LPS), as contributors to bone loss. By contrast, a healthy microbiome may protect against bone loss via production of bacterial-derived short chain fatty acids (SCFA) that stimulate bone formation (bone anabolic). Furthermore, studies suggest that estrogen deficiency and HIV infection both independently contribute to loss of gut barrier integrity. The specific interactions between gut permeability, estrogen deficiency, and HIV-infection are, however, not known. We hypothesize that bone loss in women with HIV infection is due, in part, to gut dysbiosis driving imbalance between bone catabolic and bone anabolic bacterial metabolites, and exacerbated by estrogen insufficiency/deficiency. Recent advances in high-resolution metabolomics (HRM) provide a unique opportunity to broadly explore metabolism and underlying pathways relevant to HIV and women's health. The purpose of this study is to use plasma HRM with advanced bioinformatics to interrogate the interrelationships between circulating gut bacterial-derived metabolites, bone mineral density (BMD) and biomarkers of bone formation and loss, and estrogen deficiency in women living with HIV. Aim 1 will assess the association of circulating bacterial-derived metabolites and HIV status with bone turnover and BMD, while Aim 2 will assess the interactions of estrogen on these relationships. The overall goal of this project is to obtain new insight into HIV- and estrogen deficiency-induced bone loss that will inform the design of future interventions to reduce the risk of osteoporosis and fracture among women living with HIV. This project will provide outstanding preliminary data for subsequent studies focusing on the pathophysiology of bone disease among women living with HIV and the development of new strategies to improve health and quality of life in this aging population.